1. Field of the Invention
The present invention relates to a method and an apparatus for controlling access of a font data memory used in a display control system employing a character display mode wherein an address is produced in accordance with a character code stored in a character code data memory, font data stored in a font data memory is read out in accordance with the produced address, and the font data is displayed on a display device.
2. Description of the Related Art
In a conventional display control system employing the character display mode, a font data memory is occupied during the display period. Thus, read/write access, other than display access, to the font data memory by a CPU (Central Processing Unit), a DMA (Direct Memory Access) apparatus, or the like is allowed only in a non-display period, such as a vertical blanking period or a horizontal blanking period. As a result, much time is consumed when the CPU, DMA apparatus, or the like access the font data memory.
For example, in a display control system employing both a of the character display mode and a bit map memory display mode, the font data memory is occupied during the display period in the character display mode, as stated above. Thus, the font data memory used for the display in the bit map memory display mode is occupied only in the non-display period. Consequently, much time is required for access to the font data memory.
In order to solve the above problem, there is an idea, for example, that a font data memory exclusively used in the character display mode and a font data memory exclusively used in the bit map memory mode are both provided in the display control system. However, since the font data memory is expensive, provision of two or more font data memories prevents reduction in cost and size of the system.
Under the circumstances, there is a demand for an apparatus for controlling access of a font data memory, which enables the CPU, or the like to access the font data memory with high efficiency and is used in a display control system employing the character display mode and having only one font data memory, while ensuring normal display of the display device.